Optimum antenna design for microplasma generation

In the last decade, there has been an increased interest in microplasma properties and applications. Microplasmas have been considered for use in plasma thrusters, displays, and sensors. Microplasmas are small scale plasmas which are typically generated with radio frequency power sources. The small scale of the microplasma requires equally small antennas. The sizing difference can cause difficulties in proper impedance matching for power deposition from the radio. To address this issue, different radio frequency antenna designs were investigated to generate microplasma within a glass capillary tube at the University of Alabama in Huntsville's Johnson Research Center. The experiment setup uses a multi-frequency radio, high power amplifier, and T-type matching network. The test antennas were attached to the output to provide the radio signal to the atmospheric gas inside the glass tube. The test antenna design variations include dipole and monopole configurations. Antenna materials varied in the investigation. Copper and steel wires of various gages were used to form different length coils to surround the gas tube. Shield and pin antenna designs were also explored. The standing wave ratio is used to determine the antenna's effectiveness. A low standing wave ratio value, close to 1, indicates a low voltage is reflected back to the source and the antenna has the most power being directed to microplasma generation. The best antenna designs were determined based on microplasma production and standing wave ratio value. The shield and pin antenna design successfully generated a microplasma. This result is consistent with the use of copper and stainless steel shields and a 4-40 threaded pin. The shield and pin design is the most successful because it directs the radio frequency consistently to the pin without any loss that may found in a helix design. The central pin acts as a sacrificial surface for the microplasma to generate from.